1. Field of the Invention
This invention relates to control systems for commutatorless motors.
2. Description of the Prior Art
The brushless or commutatorless motor is a variable speed motor without any commutator consisting of a combination of a synchronous motor and a power converter including controlled rectifier elements and has the advantages that
1. ANY DESIRED SPEEDS ARE OBTAINED RANGING FROM ZERO TO THE SYNCHRONOUS SPEED OR HIGHER, RESULTING IN A VERY WIDE RANGE OF SPEED CONTROL,
2. THE ABSENCE OF ANY BRUSH OR COMMUTATOR FACILITATES THE MAINTENANCE AND INSPECTION THEREOF,
3. AND REVERSIBLE DRIVE AND REGENERATIVE BRAKING ARE EASILY EFFECTED.
The power converter of the commutatorless motor is of two types: One is the cycloconverter type which employs a cycloconverter for converting an AC source voltage directly into an AC voltage of variable frequency and variable voltage, and the other is the DC link type which uses an inverter circuit whereby the AC voltage from the AC power supply is converted first into a variable DC voltage through rectifier circuit and then converted into a variable frequency AC voltage.
The power converter such as mentioned above for the commutatorless motor, which has no forced commutation circuit for commutation of the controlled rectifier elements, effects commutation of the controlled rectifier elements by taking advantage of the induced voltage produced in the armature winding of the synchronous motor. However, in view of the fact that a sufficiently high voltage to effect commutation is not induced in the armature winding of the synchronous motor at low speeds of the motor such as when the motor has just started, a commutation failure may occur in the power converter of DC link type. As to the cycloconverter type of the power converter, by contrast, no problem of the commutation failure is caused since it utilizes an AC source voltage for commutation.
As a result, in the commutatorless motor of DC link type, the rectifier circuit is subjected to inversion at each commutation of inverter circuit at low motor speeds so that the DC output current is reduced to zero thereby to assure the successful commutation of the inverter circuit. In the abovementioned case where the DC output current of the rectifier circuit is continually controlled at each commutation of the inverter circuit at low speeds of the synchronous motor, the DC portion is superimposed on the AC source current at certain frequencies, i.e. 50/6 and 50/12 cycles of the operating frequency of the synchronous motor, with the result that the DC excitation in the transformer core interposed between the AC power supply and the rectifier circuit is affected or distorted by the DC portion. The foregoing description is based on the assumption that the frequency of the AC power supply is 50 cycles.
This effect of the direct current excitation in the transformer core often causes the overheating of the transformer or breakdown thereof under heat or unwanted noises due to an increased temperature and exciting current. Therefore, it is very important to prevent the DC portion from being superimposed on the AC source current.